For design and operation of multiphase pipelines the knowledge of the volume of the liquid phases may be crucial for estimating the pressure drop in the lines and for design and operation of the multiphase line and the receiving facilities. Especially for gas condensate pipelines, where the liquid content in the line may depend strongly on the flow rate, the pipeline diameter selection, the design of the slug catcher (volume, and drainage rates) and the operational procedures can be optimised if accurate estimates for liquid volumes in the lines at various flow rates are available. For gas condensate production systems production scenarios as start-ups, production increases, pigging and tail end production must be designed for. Reliable information and measurements of the liquid volumes in other lines in the same parameter range as the line to be designed can be used to tune and verify a predictive multiphase flow model to be used as engineering tool for the line in question. Measurements of liquid volumes in the line, when in operation, can be used to optimise the operations and, thereby improving product quality and production regularity.
Fluids from reservoirs or production facilities will be transported in production pipelines and/or export pipelines to receiving facilities for further refinements. The flow in both the production and the export lines may be multiphase flow. i.e. a gas phase and/or a hydrocarbon liquid phase and/or a water phase and/or solid phases may be transported in the line.
The phases will be transported with different average velocities through the line. Several transport mechanisms for the phases can be seen: transport as liquid film, transport as droplets in the gas or in other liquid phases, transport as liquid slugs or waves, transport of gas as bubbles, transport of particles in the liquid etc. Condensation/evapo-ration/solidification can also occur during transport.
The average velocity of each phase through the line depends on the transport mechanisms seen in the line. The velocities along the line vary with pipeline profile, diameter changes, pressures, temperatures, condensation and rate changes (incoming pipelines in networks). The differences in phase velocities will give different phase volume fractions in the cross sections along the line.
The total phase volumes in the line are directly calculated from the local phase fractions along the line. Due to the complicated transport mechanisms the local phase volume fractions cannot easily be estimated. Simulation tools incorporating multiphase flow models are therefore used to predict the phase fractions along the line, the total volume of the phases in the line, and pressure drops. A standard tool here is the dynamic multiphase code OLGA 2000, but steady state pipe simulators can also be used.
The simulation tools are used as engineering tools for the design and for operational support. The multiphase flow models are based on correlations, which are usually based on small-scale laboratory experiments. Field measurements are thus used to verify the models. Field measurements can also be used to tune the flow models for specific lines. Field measurements of total phase volumes for multiphase lines are very important for verification and tuning of the models.